1. Field of the Invention
The present invention relates to a projection type image display device, such as a liquid crystal projector, including a color synthesizing optical system that uses a dichroic membrane, or film.
2. Description of the Related Art
Conventionally, various devices have been proposed as a projection type image display device, such as a liquid crystal projector, including a color synthesizing optical system.
A conventional projection type image display device will now be described with reference to FIG. 8. In FIG. 8, white light emitted from a light source 201 is transformed into substantially parallel rays of light by a parabolic mirror 202, a reflecting mirror (not shown), and a lens group (not shown).
The substantially parallel rays of light are subjected to color separation by dichroic mirrors 203 and 204, highly reflecting mirrors 205, 205 and 207, and a lens group (not shown), and are condensed to liquid crystal panels 208, 209 and 210 disposed for each color. Each color light that has entered the liquid crystal panels 208, 209 and 210 is then modulated by the liquid crystal panels 208, 209 and 210 displaying an image, thereby generating spatially modulated light according to the display image.
Each color modulated light is subjected to color synthesis by a color synthesizing dichroic prism 211, and is enlarged and projected onto a screen 213 through a projection lens group 212. Meanwhile, a group of positive refractive lenses 214, 215 and 216 are disposed between each liquid crystal panel 208, 209 and 210 and the dichroic prism 211 in order to reduce the optical system in size. Therefore, each color modulated light in the form of substantially parallel light that has passed through each liquid crystal panel 208, 209 and 210 is effected by the condensing (converging) action of the positive refractive lenses 214, 215 and 216, and is made non-parallel. This non-parallel light strikes the color synthesizing dichroic prism 211.
Therefore, disadvantageously, for the example, the incidence angle of the light upon the dichroic membrane has a difference between the right and the left of the image, and thereby a difference arises in the spectral/transmitted cutoff wavelength in the dichroic membrane. As a result, in each color light, nonuniformity (color unevenness or brightness irregularity) in the brightness level occurs in the right and the left of the screen.
As a solution to the color unevenness, a countermeasure has been made to form a so-called gradient membrane, or film, as the dichroic membrane in which thickness or a refractive index slantingly varies in a specific direction.
However, there is a problem in that, usually, color unevenness can be simply corrected only in one direction in the thus formed gradient membrane, so that color unevenness in each direction cannot be sufficiently corrected, and, in addition, a color synthesizing prism becomes expensive due to the formation of the gradient membrane.